Advanced Topic 394 of 1102 Thermodynamics & Energetics
Fugacity and Activity
Effective pressure and concentration that correct for non-ideal behaviour.
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In-depth explanation of Fugacity and Activity
Fugacity and activity are thermodynamic corrections that allow ideal-looking equations to be applied to real gases and real solutions. Fugacity is the effective pressure of a real gas, defined so that the chemical potential takes the same form as for an ideal gas, while activity is the corresponding effective concentration in a nonideal solution. The ratio of fugacity to pressure is the fugacity coefficient, and the ratio of activity to concentration is the activity coefficient, both of which approach one under ideal conditions. These coefficients account for intermolecular interactions and deviations from ideality at high pressure or high concentration. Using them keeps equilibrium calculations rigorous for nonideal systems.
Going deeper: Fugacity and Activity
Key ideas: Replace pressure with fugacity and concentration with activity in equilibrium expressions to keep them rigorous. For dilute solutions activity is close to concentration, and for ions the Debye-Huckel law estimates the activity coefficient.
Common mistakes and connections: Students often apply ideal expressions at high ionic strength or high pressure and get errors. Pure solids and liquids have activity of one. This idea corrects equilibrium constants and cell potentials for real behavior.
Visualization of Fugacity and Activity
Real gases deviate from ideal behaviour at high pressure; fugacity corrects the pressure, and the compressibility factor Z shows the size of the deviation. The interactive lesson shows this as a live animation or chart.
Worked example and use case
In concentrated brine the activity coefficients of the ions are far below one, so the effective concentration is much lower than the measured concentration.
Frequently asked questions about Fugacity and Activity
What is Fugacity and Activity?
Effective pressure and concentration that correct for non-ideal behaviour.
Is Fugacity and Activity a beginner, intermediate or advanced topic?
Fugacity and Activity is a advanced topic in Thermodynamics & Energetics. It builds on beginner and intermediate topics, so review those first if needed.
What should I learn before Fugacity and Activity?
Study these earlier topics in Thermodynamics & Energetics first: Partition Function, Boltzmann Distribution, Chemical Potential.
What comes after Fugacity and Activity?
The next topics in Thermodynamics & Energetics are Gibbs Phase Rule and Clapeyron Equation.
Can I practise Fugacity and Activity with quizzes?
Yes. The interactive lesson includes a self-check question on Fugacity and Activity, and the Thermodynamics & Energetics quiz draws on every topic in this area.
Where Fugacity and Activity fits in Thermodynamics & Energetics
Fugacity and Activity is topic 66 of 91 in Thermodynamics & Energetics.